The present invention relates generally to a method and apparatus for increasing levitation pressure in superconducting magnetic bearings. More particularly, the present invention relates to a method and apparatus for producing large levitational pressures in superconducting magnetic bearings including a permanent magnet and a superconducting material. One preferred embodiment of the invention utilizes large flux pinning forces in superconductors to achieve compression of the flux emerging from permanent magnets or other magnetic flux producing devices. The compressed flux produces a large magnetic field, which results in large magnetic pressures.
The stable levitation of a permanent magnet over a sample of ceramic superconductor in a container of liquid nitrogen is well known and represents the rapidly advancing technology associated with high-temperature superconductivity. The use of this levitation force in magnetic bearing applications is thought to comprise one of the earliest applications of the high-temperature superconductors, because for these applications there is generally no need for current to flow between individual grains of the high temperature superconductor. Rather, the magnetization properties within individual grains are important, and such magnetization has exhibited some very high values since the early days of the high temperature superconductor discoveries.
The levitation pressure P of a magnetic interaction on any surface can be calculated to a first approximation by EQU P=(B.sub.t.sup.2 -B.sub.n.sup.2)/2.mu..sub.o, (1)
where .mu..sub.o =4.pi..times.10.sup.-7 Tm/A(T.sup.a /Pa) is the permeability of free space, B is the magnetic induction (typically referred to herein as the magnetic field), subscript "t" refers to the tangential field component, and subscript "n" refers to the normal field component. The magnetic induction is related to magnetic flux .PHI. according to the relation: EQU B=.PHI./A, (2)
where A is the cross-sectional area of any imaginary or real surface through which the magnetic flux passes.
It has conventionally been believed that the maximum levitation pressure obtainable in a permanent magnet/high temperature superconducting material system is given by the value of B measured at a pole face of the permanent magnet. The measured pole face field is of the order of 0.5 T or less for most geometries and state-of-the-art permanent magnets. The associated magnetic pressure of a 0.5 T field is 100 kPa (1 arm or .about.15 psi). This magnetic pressure is approximately equal to that produced in conventional gas-film bearings, but is significantly less than that of liquid film bearings or roller bearings. Nevertheless, for cryogenic applications, where mechanical wear is very high and where thermal losses associated with contact produce large refrigeration penalties, gas-film bearings are presently in use or development. It is for these applications that magnetic bearings with their total absence of contact have an immediate advantage. Further, high temperature superconductor magnetic bearings can even provide advantages over state of the art magnetic bearings if larger levitation pressures can be achieved.
The conventional wisdom previously described regarding the maximum levitational pressure obtainable has been shown herein to be an artificially low level which has been overcome by the instant invention. The unique method and apparatus of the invention uses flux compression to generate larger levitation pressures than previously believed practical.
It is therefore an object of the invention to provide a novel method and apparatus for obtaining maximum possible levitational force in a system comprising a magnet and a high temperature superconducting material.
It is a further object of the invention to provide an improved device and method in which a superconducting material structure is used to compress flux emerging from a permanent magnet to provide high magnetic field strength in one or more confined regions.
It is a still further object of the invention to provide a novel superconducting magnetic bearing and method of use which is passively stable.
It is yet another object of the invention to provide an improved superconducting magnetic bearing and method of use which compresses magnetic flux and generates large levitation forces using a substantially horizontal, diamagnetic superconducting material.